Apparatus and methods for the forge welding together of a pair of metal parts or the edge portions of a single part folded to form a tube in which the surface portions of the metal part or parts to be welded together are advanced toward a weld point with a gap therebetween are heated by high frequency electrical currents supplied to the surfaces by way of contacts at opposite sides of the gap and contacting the metal part or parts in advance of the weld point are well known in the art. See, for example, U.S. Pat. Nos. 2,821,619; 2,873,353; 2,886,691; 2,898,440; 2,992,319; 3,047,712; 3,056,882; 3,375,344; 3,391,267 and 4,241,284 which are incorporated herein by reference.
In the high frequency prior art heating apparatus, the downstream edges of the contacts extend at substantially 90.degree. to the paths of advance of the metal to be heated. In addition, the contacts are tilted so that only a small portion of the contact faces adjacent to the downstream edges contacts the metal to be heated. Thus, there is a very high current density at such portions of the contact faces. In addition, as will be further explained hereinafter, with high frequency currents and with the downstream edges of the contacts extending at 90.degree. with respect to the path of the metal to be heated, a large proportion of the current leaves the contacts at the portions of the contact face nearest the heating current path in the metal. Therefore, the current density at the portions of the contact faces nearest the heating current path in the metal is much higher than elsewhere at the current faces.
The use of high frequency electrical currents, i.e. currents of a frequency of 10 KHz and higher, and particularly of 400 KHz and higher, for the heating of the surfaces to be welded together has certain well-known advantages as compared to the use of direct currents or currents of 500 Hz or lower. For example, "skin effect" causes most of the current to flow at the surface of a part where it is most useful, and such effect increases with frequency. Furthermore, the surfaces contacted by the contacts need not be clean, e.g. they can have scale or oxides thereon.
When direct currents or low frequency currents, i.e. 500 Hz or less are used, there is little skin effect. In addition, the current path, when proximity effect is not involved, is determined primarily by the resistance of the path because the inductive reactance of the current paths is zero or small. Thus, the current density per unit area of the face of the contact which contacts the part is substantially uniform. Accordingly, to decrease the current density, one merely has to increase the contact area.
Current density where the contact engages the metal part is important in at least two respects. Thus, if the density is too high, undesirable melting of the metal can occur in the area of contact or close thereto. Such melting is not desirable for forge welding because it defaces the metal and can cause hardening of the metal by self-quenching after it cools. Even if there is no melting, the metal can become discolored, have bum marks, cause self-quenched hardened areas or melt metal of the contacts onto the metal part which contact metal must be removed.
In actual practice with high frequency currents, the current can be several thousands of amperes with welding speeds, i.e. advancing speed, of 25-500 ft/min. It has been found that with such large currents, and hence, a high contact current density, the problems mentioned hereinbefore have been encountered but have been tolerated for some purposes because of the production speeds available.
Because of various pieces of equipment needed for mechanical reasons, e.g. forming rolls, forge welding pressure rolls, etc., the space available for the contacts and their mountings is limited. However, while increasing the size of the contacting face of the contacts causes some improvement with high frequency currents, there is a need for further improvement. Even with an increase of the size of the contacting face and high frequency currents, most of the current concentrates at the portion of the face nearest the metal surface to be heated and particularly, at the downstream corner nearest the surface being heated.